But is getting a free activation code legitimate? Is it safe? And if you cannot find a working code, what are the real alternatives?

If you don't want to pay for a license, there are highly-rated, (open-source) alternatives that provide full functionality:

The free version of Duplicate Sweeper is – it is a scanner. It will:

: If the free version of Duplicate Sweeper doesn't meet your needs, consider alternative duplicate file finders. Some other tools offer robust free versions with generous limits or completely free functionality.